Abstract: This paper dealt with the research on the activities of pheny-lalanine ammonialyase (PAL) in the poplar clones of Aigeiros section, including I-63, I-69 and I-72. The maximal activities of PAL in the barks of their 1-year-old dormant stems were seen in the top of stem without exception, and those of them were similar in main. But in their middle and basal parts the PAL activities varied with clones significantly, among them that of I-69 poplar was the highest, and this difference continued all the way up to their sprouting. The photomicrographes of their sprouting stem sections showed that the vessel elements in the stem sprout of I-69 poplar were well-developed and are much better than those in I-72 poplar. This phenomenon was in accordance with the difference between their PAL activities, which indicated that the PAL activity was closely related to the lig-nif ication of the poplar stem. The sequence of PAL activities in the leaves of various clones during the developing process from their winter buds to the complete leaves mainly coincided with those in the barks of dormant stem and stem sprout of corresponding clones.